A computed tomography (CT) scanner generally includes a rotating gantry rotatably mounted to a stationary gantry. The rotating gantry supports an X-ray tube and is configured to rotate around an examination region about a longitudinal axis. A detector array is located opposite the X-ray tube, across the examination region. The X-ray tube is configured to emit poly-energetic ionizing radiation that traverses the examination region (and a portion of an object or subject therein) and illuminates the detector array. The detector array includes a one or two dimensional array of detector pixels that detect the radiation and that generate signals indicative thereof. Each pixel is associated with a readout channel, which is used to convey a corresponding signal for further processing. A reconstructor reconstructs the processed signals, producing volumetric image data indicative of the examination region.
For spectral CT, the detector pixels have included direct conversion detector pixels. Generally, a direct conversion pixel includes a direct conversion material (e.g., cadmium telluride (CdTe), cadmium zinc telluride (CZT) etc.) disposed between a cathode and an anode, with a voltage applied across the cathode and the anode. X-ray photons illuminate the cathode, transferring energy to electrons in the direct conversion material, which creates electron/hole pairs, with the electrons drifting towards the anode. The anode, in response, produces the electrical signal output by the detector array. An amplifier amplifies the electrical signal, and a pulse shaper processes the amplified electrical signal and produces a pulse having a peak amplitude or height that is indicative of the energy of the detected radiation. An energy discriminator compares the height of the pulse with one or more energy thresholds. For each threshold, a counter counts the number of times the pulse height crosses the threshold. An energy-binner bins the counts in energy-ranges, thereby energy-resolving the detected radiation. The reconstructor reconstructs the binned signals using a spectral reconstruction algorithm.
Direct conversion material such as CdTe and CZT tends to produce a low frequency electrical current when irradiated with X-rays, which results in a baseline shift of the signals output by the detector pixels. Unfortunately, the baseline shift shifts the pulse output by the shaper, which can lead to erroneous binning of the detected radiation into incorrect energy bins as the discriminator thresholds remain static. There are two main components of this low frequency electrical current, namely dark current and persistent current. The dark current is a DC component that depends on the detector material and the bias voltage and usually does not change during an acquisition interval. This component can simply be corrected with a static bias compensation, which injects the same amount of current with the opposite sign to the input of the amplifier. The persistent current (PC) is caused by trapping (in the direct conversion material) of holes of the electron-hole pairs. Because of the positive potential of the trapped charges, electrons are injected into the bulk material and move to the anode instead of recombining with the holes. The resulting slowly varying current can be very strong and can exceed the photo current (the amount of charge directly generated by photons) by two orders of magnitude. This persistent current causes significant signal degradation and may generate unacceptable image artefacts if left uncorrected. Unfortunately, the persistent current dynamically changes and cannot simply be compensated with a static signal of the opposite sign like the dark current.